A selective NOx reduction catalyst (also referred to in the following simply as an “NOx catalyst”) for effecting the reductive purification of the NOx in an exhaust gas may be provided in the exhaust gas line of an internal combustion engine. For example, it is known that an NOx catalyst may be formed by supporting at least one active component that exhibits a selective reduction activity for NOx, e.g., Fe, Cu, and so forth through ion-exchange, within the pores of a zeolite, as in the art described in Patent Document 1.
In addition, with the NOx catalysts disclosed in Patent Documents 2 and 3, an improvement in the purification rate for the NOx in exhaust gas is devised by the disposition in the exhaust gas line of a suitable combination of a first zeolite provided by ion-exchanging a zeolite crystal with Cu and a second zeolite provided by ion-exchanging a zeolite crystal with Fe. Patent Document 4 discloses an NOx catalyst formed so as to have, for a lower layer, a layer in which a zeolite crystal has been ion-exchanged with Fe, and, for an upper layer, a layer in which a zeolite crystal has been ion-exchanged with Cu.
[Patent Document 1] Japanese Patent Application Laid-open No. 2008-221203
[Patent Document 2] Japanese Patent Application Laid-open No. 2010-499
[Patent Document 3] Japanese Patent Application Laid-open No. 2011-125849
[Patent Document 4] Japanese Patent Application Laid-open No. 2013-13894
[Patent Document 5] Japanese Patent Application Laid-open No. 2011-167690